A technique in which a display device such as a projector detects the position of a digital pen (pointer) on a screen and projects an image (hand-drawn image) corresponding to a trace of the digital pen is known. This function is called an electronic blackboard function or electronic whiteboard function. A projector with the electronic blackboard function divides the screen into a plurality of areas, taking advantage of the large size of the screen, and draws hand-drawn images separately in the individual areas, using a plurality of digital pens (PTL 1 and PTL 2, for example).